522 TEXT-BOOK OF PHYSIOLOGY. 



tinuous with that lining the central canal of the cord. Beneath this is a 

 layer of gray matter. 



The pons varolii comprises in a general way that portion of the central 

 nerve system situated between the medulla oblongata and the crura cerebri. 

 The ventral surface is convex from side to side ; the lateral surface, owing to 

 the convergence of the fibers of which it is composed, is contracted to form the 

 middle peduncle of the cerebellum; the posterior surface is flat and forms 

 the upper half of the floor of the fourth ventricle. The pons consists of 

 white fibers and gray matter supported by connective tissue and neuroglia. 

 Transverse sections of the pons show that it is divided into an anterior 

 or ventral, and a posterior or dorsal portion, the latter being usually termed 

 the tegmentum. 



The ventral portion consists for the most part of white fibers, arranged 

 longitudinally and transversely (Fig. 240). The longitudinal fibers are 

 largely continuations of the pyramidal tracts, or the fibers composing in part 

 the anterior pyramid of the medulla. In the lower part of the pons these 



fibers are compactly arranged, but at higher 

 levels they are separated into a number of 

 bundles by the interlacing of the transverse 

 fibers. The transverse fibers are divided into 

 a superficial and a deep set. Among these 

 fibers are groups of nerve-cells which collec- 

 tively are known as the nucleus pontis. Some 

 of the transverse fibers, especially the super- 

 ficial ones, are commissural in character i.e., 



Fto.*^-T*HSenoK iH. the y con r ne u ct corresponding parts of the gray 

 PONS THROUGH ITS MIDDLE FOR- matter of the lateral halves of the cerebellum; 

 TION, SHOWING THE RELATION others coming from the gray matter of the 

 T F , T s H Co*L E po V sED TRA T/ F DoS cerebellum cross the median line and terminate 

 longitudinal fasciculus. L.c. and around the cells of the nucleus pontis; others 

 ^LOCUS cemleus. L.f. Lateral again are connected with the gray cells of the 



same side. Through the intermediation of 



the nucleus pontis and certain of the longitudinal fibers of the pons, the 

 cerebellum is brought into relation with the cerebrum. 



The dorsal or tegmental portion consists of: (i) The fillet; (2) the formatio 

 reticularis; (3) the medial longitudinal bundle; (4) groups of efferent and 

 afferent nerve-cells. 



The fillet or lemniscus in this region is divided into a mesial and a lateral 

 portion. The fibers of the mesial portion are partly the axons of the nerve- 

 cells of the gracile and cuneate nuclei of the opposite side of the medulla, and 

 partly of the axons of the sensor nerve-cells of the afferent cranial nerves with 

 the exception of the auditory. The fibers of the lateral portion are mainly 

 the axons of the cells in the floor of the fourth ventricle around which the 

 auditory nerve-fibers end. They are therefore a continuation of the acoustic 

 tract. * 



The formatio reticularis is a continuation of that of the medulla. 



The medial longitudinal bundle is a band of nerve-fibers, triangular in 

 shape, placed on either side of the median line just beneath the floor of the 

 fourth ventricle and the aqueduct of Sylvius. It consists of both afferent 



